The following U.S. patents are believed to represent the current state of the art: U.S. Pat. Nos. 6,817,995; 6,562,023; 6,488,674; 6,254,589; 6,190,372; 5,993,437; 5,667,490; 5,632,729; 5,562,618; 5,531,723; 5,290,277; 5,188,607; 4,950,255 and 4,769,017.